First, I want to welcome you to Yorkie Talk. You will find so much information here. We do have a wonderful breeders section.
To be honest, some females are very difficult to breed. She may just be scared. You may have to hold her firmly still and let the male mount her but sometimes that does not work if she is growling and snapping. The only other thing would be to muzzle her and hold her by the shoulders while the male breeds her. I have had some females that wanted nothing to do with the male and I know they would were ready because they would be flagging but they would get scared when the male mounts them. On a few occasions I have had to muzzle a virgin female and once the first tie is accomplished the female would do just fine.